The human body is predominantly comprised of water. The water phantom is especially well suited for determining the effect of the radiation on the human body. A water phantom may measure the radiation penetrating a container filled with water. The radiation may be measured by a detector or an array of individual detectors that can be positioned inside the water in various measurement positions by a positioning device.
The local ionization of the water caused by radiation can be measured, and a conclusion can be drawn about the applicable local energy input of the radiation to the water. By positioning the detector in various measurement positions along the radiation direction, a radiation profile of the penetration depth may be determined. By positioning the detector in various measurement positions on a straight line perpendicular to the radiation direction, the radiation profile transverse to the propagation direction may be determined. The radiation profiles and other parameters that characterize the radiation may hereinafter be referred to by the general term radiation “parameters”.
The radiation parameters are ascertained at regular time intervals, typically once a day. The radiation parameters are compared with set-point values in order to assure constant quality of the radiation dose and, if needed, to recalibrate the radiation source. Often, comprehensive regular monitoring of the radiation parameters is prescribed by law. Monitoring operations can take from half a day to several days to complete.
Depending on the radiation source used, the radiation strikes the water phantom in either a horizontal or vertically downward-oriented direction. Water phantoms usually have a container, open at the top, for the water. In the case of a vertically downward-oriented radiation direction, the radiation penetrates the water directly from above. In that case, the penetration depth of the radiation into the water is determined by the spacing of the detector from the water surface. This spacing must therefore be known as accurately as possible to avoid errors in measurement of the radiation parameters, and especially of the radiation profile as a function of the penetration depth. For a horizontal radiation direction, the radiation first penetrates the outer wall of the container before penetrating the water in the container. The influence of the outer wall on the radiation is taken into account in ascertaining the radiation parameters. If the water level is too low, there is a possibility that, with radiation extending close to the water surface, scattering of the radiation will occur at the water surface. Scattering may cause the radiation parameters to be incorrectly ascertained. The positioning device must also be oriented relative to the radiation direction, so that the radiation parameters can be spatially associated as accurately as possible with the radiation.
Prior to measuring the radiation, the positioning device is oriented with respect to the water surface, to make it possible to use spacing, or distance, of the detector from the water surface into account in ascertaining the radiation parameters. Typically, the container of the water phantom is block-shaped, and, with the aid of the positioning device, the detector can be moved to the various measurement positions. The motion is substantially horizontal in two directions of motion perpendicular to one another along the lower edges of the container, and substantially vertically in a further direction of motion along the sides of the container. When the positioning device is oriented such that the two essentially horizontal directions of motion are exactly horizontal and thus parallel to the water surface or, alternatively, the vertical direction of motion is exactly plumb and hence perpendicular to the water surface, then, by determining the distance of the detector from the water surface at an arbitrary measurement position, the distance of the detector from the water surface at any other measurement position is also ascertainable. With a horizontal radiation direction, the radiation extends parallel to the water surface, while with a vertical radiation direction the radiation extends perpendicular to the water surface. Because of the orientation of the positioning device with respect to the water surface, the positioning device is also oriented relative to the radiation direction.
Frequently, the positioning device is solidly connected to the container, and the three directions of motion extend precisely parallel to the edges of the container. The container, together with the positioning device, is tiltably supported about two horizontal axes. The positioning device can be oriented relative to the water surface, using at least two measuring scales located vertically on the sidewalls, for ascertaining the water level. When the water level is being read off from one of the measuring scales, an error in measurement can easily occur, for instance from parallax that is dependent on the viewing angle or from a meniscus that the water forms with respect to the side walls of the container. The container is supported on feet that can be adjusted in height by means of a screw thread. Water phantoms are also known whose orientation is adjustable by the user, such as with electric motors.
In a water phantom product known as “Blue Phantom,” made by Scanditronix Wellhöfer GmbH (Schwarzenbruck, Germany), the water level may be measured by a spacing sensor, located above the water surface in a corner of the container. The spacing sensor uses an echo pulse principle. For the “Blue Phantom” water phantom, a water reservoir communicates with the container via a supply line, and a water pump is regulated as a function of the water level ascertained by the spacing sensor, such that the water maintains an intended level despite any possible evaporation of the water. Adjusting screws provide for manual orientation of the positioning device relative to the container.
The orientation of a positioning device with respect to the water surface is complex. There is the risk that the orientation will be done imprecisely, causing the radiation parameters to be ascertained incorrectly. Ascertaining and adjusting the orientation of the positioning device by the user is time-consuming. The time required for this adjustment reduces the time available for the diagnostic or therapeutic use of the particular radiation source.